C4GP 1.6 VTR+ Electrical gremlins

[Oletimer]

I've had this car about 3 months, until now it's been ok. (Not as lively as my old C4GP 2.0L Exclusive).

2 days ago I noticed when switching off the ignition that the lights on my temp control panel and handbrake light stayed on -
the lights on the temp control panel 'flickered'.
Got out of the car and locked it via the fob, the lights inside stayed on.
Went thru the routine 3 more times and found that if I left the unlocked those lights stayed off.
Left it overnight, came to it this morning and nothing works, can't get in with the fob, can only open the front passenger door with the key.
No other doors will open, even with internal handles.
Battery is completely flat.
I have a battery charger to get the battery back up to par, but that doesn't sort out the underlieing problem.
Anyone have any ideas what the problem might be? I have been told it might be a fuse box or wiring fault.
Apparently, it's a common problem with C4's between 2004 and 2007/8.
HELP

George